The well-known principles underlying distance relaying are that the impedance measured by a relay is proportional to the distance between the relay and a fault on the protected line segment, and that, by measuring the impedance, one can determine whether or not the line is faulted. Distance protection systems have been applied extensively because (a) they can work with only local voltage and current measurements; (b) they can be used in various pilot protection schemes when a communication channel is available; and (c) they can provide coordinated backup protection for adjacent lines. The estimated fault location can be distorted, however, by inaccurate measuring devices and line parameter settings, and by zone settings that become inappropriate due to the dynamic nature of the power system (e.g., the original zone settings may need to be adjusted in view of changes in the load, generator and system topology).